The present invention generally relates to optical circuits. More particularly, the invention relates to multi-layer routed fiber optical circuits and methods for fabricating multi-layer routed fiber optical circuits.
The use of optical fibers for high-volume high-speed communication and data transfer is well established. As the volume and speed of transmitted information grows, the desire for systems using multiple optical fibers has increased. The rapid increase in communication speeds has created a demand for optical circuits to enhance or replace electrical circuits in many different applications. Optical circuits have bandwidth capabilities orders of magnitude beyond electrical circuits, and are inherently immune to electrical interference.
Fabrication of fiber-based optical circuits is known in the art. For example, it is known that optical circuits may be fabricated by positioning optical fibers in a particular pattern and adhesively bonding or embedding the fibers using pressure sensitive adhesives (PSA) or partially cured monomers coated on laminating films. The adhesive and optical fiber assembly can then be further protected by, for example, applying a cover layer, curing the adhesive, or flood coating and curing. Other optical circuits can be fabricated by patterning optical fibers on an adhesive coated film and laminating the assembly to the surface of an electrical circuit board. Still other optical circuits are constructed by embedding optical fibers or waveguides within a circuit board. In each case, the finished assembly consists of optical fibers or waveguides held firmly in place in an intermediate layer of a multi-layer assembly.
When fabricating optical circuits, especially those which consist of optical fibers laminated between two flexible substrates (commonly referred to as “flex foils”), it is customary to generate the desired circuit pattern in a single layer. In the event that a large number of circuit lines are required, and when the circuit layout permits, a higher density design may be achieved by stacking two or more flex foils into a single lamination, thereby forming a multi-layer optical circuit. However, when the circuit layout becomes very complex, it becomes more difficult to fabricate a circuit in multiple layers by simply stacking discreet circuits for a number of reasons. Specifically, it is likely that the circuit design will require fibers from one layer of the circuit to be routed with fibers on another layer of the circuit. It is also possible that fibers from multiple layers will need to terminate into a single connector. It may also be required that fiber lengths be optimized to minimize skew between channels; such length optimization in a high density interconnect design with adjacent connectors positioned very close together may make it very difficult or impossible to equalize lengths on a single layer.
There are other problems associated with positioning optical fibers or waveguides in an intermediate layer of a multi-layer assembly. For example, terminating optical fibers or waveguides positioned in an intermediate layer of a multi-layer assembly can be problematic, as the optical fibers or waveguides are not readily accessible for connector mounting and polishing. Interlayer optical coupling is also difficult, because each optical circuit layer is independently formed prior to lamination. The optical circuit designer must therefore resort to exotic measures to couple light into and out of waveguides or optical fibers buried in an inner board layer. Light coupling generally involves directing light into the board at an angle orthogonal to the surface of the board, and then somehow turning the light 90 degrees and coupling into the waveguide. Light coupling measures require the use of angled waveguides, inclusive of mirrors, lenses, etc. Such assemblies are difficult to assemble, increase costs, and usually increase signal loss in the device.
Still other problems associated with positioning optical fibers or waveguides in an intermediate layer of a multi-layer assembly include, for example, microbending stresses and associated optical losses that occur as the circuit layers are laminated together and optical fibers cross over each other due to requirements of the circuit pattern. Also, fibers rigidly held in such optical circuit assemblies may exhibit increased bending loss caused by temperature-induced stress.